“Using pesticides at work 'increases risk of Parkinson's three-fold,'” according to The Daily Telegraph, which has reported on new research into Parkinson’s disease and a number of occupations. The newspaper said that overall, the pesticides examined increased the risk by 80%, with three chemicals, including the insecticide permethrin and the weedkiller paraquat, increasing risk three-fold.

The research behind the story compared information on 519 patients with Parkinson’s disease and 511 healthy people. It found that 44 of the patients with the disease and 27 of the healthy volunteers were exposed to pesticides at work. This research suggests that occupational exposure to certain pesticides can increase risk of Parkinson’s disease, but these results will need interpretation alongside other similar research and in light of its limitations.

It is important to note that occupational exposure to pesticides was assessed but not other methods of exposure, such as gardening as a hobby, living near somewhere that pesticides are used, wearing pesticide-treated clothing or dietary intake. Without looking at these factors it is not possible to say whether exposure through these routes affects risk of parkinsonism (any disorder manifesting the symptoms of Parkinson’s).

Where did the story come from?

This study was carried out by Dr Caroline Tanner and colleagues from The Parkinson’s Institute in California and other research centres in the US and Canada. The study was funded by an unrestricted grant from a group of manufacturers of welding products. One of the study’s authors had received fees for providing expert testimony in cases related to Parkinson’s disease in welders. The study was published in the peer-reviewed medical journal Archives of Neurology.

What kind of scientific study was this?

This was a case-control study looking at the relationship between occupation, exposure to various chemicals and risk of Parkinson’s disease. Some past research has suggested that a number of occupations and chemicals might be associated with an increased risk of Parkinson’s disease, but the associations found have not been consistent across studies. The occupations assessed in this study included those in the fields of agriculture, education, healthcare, welding and mining, while the chemicals assessed included solvents and pesticides.

The researchers enrolled 519 people who had presented parkinsonian symptoms at one of eight movement disorder clinics in the US between July 2004 and May 2007. To be enrolled as cases in the study individuals needed to have a tremor at rest or slow movement plus at least one more sign of parkinsonism (rigidity of muscles, slow movements and problems with balance and coordination) and a diagnosis within the previous eight years. Age at onset of parkinsonism was recorded and type of parkinsonism was assessed. People with dementia, or whose parkinsonism had a known cause, were not eligible to participate.

Control individuals were matched to cases’ age, gender and recruitment location. The 511 control individuals recruited were mainly the cases’ non-blood relatives (excluding spouses) and acquaintances, who did not have typical signs of Parkinson’s disease. Workmates were not eligible to act as controls.

Cases and controls completed telephone interviews to provide information about lifetime use of tobacco, alcohol and caffeine, any head injuries resulting in loss of consciousness or medically diagnosed concussion and previous occupations. Regular use of tobacco, caffeine or alcohol was defined as use for at least six months. Information was collected on each job held for three months or longer, including details of the industry, location, processes, materials and job tasks.

Researchers only looked at exposure that occurred before diagnosis for cases and exposure of controls before the average (median) age of diagnosis for cases of the same age and gender. To reduce the chance of bias the researchers inputting the information from the interviews did not know whether they were interviewing cases or controls.

The researchers then compared occupations and occupational exposures between cases and controls. They also looked at the association between these factors and early onset parkinsonism (age ≤50 years), and with specific subtypes of parkinsonism, including Parkinson’s disease, atypical parkinsonism, and postural instability and gait difficulty. Their analysis was adjusted for factors that could influence results, such as age, gender, race or ethnicity, head injury, duration of job or task and total lifetime use of tobacco, caffeine and alcohol use.

What were the results of the study?

In total 91% of cases and controls could be included in the main analysis. The majority of cases were white males and the majority had Parkinson’s disease (96.9%). Having ever smoked was associated with a reduced risk of parkinsonism, while having ever drunk coffee was also associated with a reduced risk of parkinsonism, although this reduction did not quite reach significant levels.

The researchers found that working in agriculture, education, healthcare or welding was not associated with increased risk of parkinsonism. Working in legal, construction and extraction, or religious occupations was associated with an increased risk of parkinsonism. Personal care and service workers, food preparation workers and military tactical weapons specialists were at a reduced risk of parkinsonism. However, these associations did not remain significant after adjusting for length of time in the profession.

There was no association between use of solvents, painting, soldering, machining, using glue or adhesives, woodworking and stripping wood or paint and risk of parkinsonism.

People who used pesticides at work were at increased risk of parkinsonism: 8.5% of parkinsonism cases using pesticides compared to 5.3% of controls (odds ratio [OR] 1.90, 95% confidence interval [CI] 1.12 to 3.21). This included people who used any of the eight pesticides thought to be potential risk factors (OR 2.20, 95% CI 1.02 to 4.75). When looking at individual pesticides, only 2, 4-Dichlorophenoxyacetic acid was associated with a significant increase in risk of parkinsonism (OR 2.59, 95% 1.03 to 6.48). Other pesticides were rarely used. Use of paraquat and permethrin were associated with increases in risk that did not reach statistical significance.

None of the specific occupations, tasks or task-related exposures were associated with younger diagnosis of parkinsonism at a younger age (≤50 years) or atypical parkinsonism. Having ever worked in business and finance, legal occupations, construction and extraction or transportation and material moving was associated with postural instability and gait difficulty subtypes of parkinsonism. However, these associations did not remain significant after adjusting for the duration the role was held.

What interpretations did the researchers draw from these results?

The researchers conclude that use of pesticides was associated with an almost 80% increase in the risk of parkinsonism. They say that this link supports the possibility that Parkinson’s disease is caused by exposure to toxic chemicals.

What does the NHS Knowledge Service make of this study?

There are a number of limitations to their study, including:

As with all studies of this type, it is possible that the association seen is not due to the suggested risk factor itself, but instead to some other factor that is associated with it. The researchers did take some factors into account, but there may still be other factors that are having an effect.

Exposures had to be estimated retrospectively based on interviews. This may result in inaccuracies, particularly if individuals believe their work exposures to be related to their parkinsonism. The researchers attempted to minimise biasing participants by not mentioning exactly which risk factors were being assessed, with the interview questionnaires covering a complete life history of occupations and tasks.

The study carried out multiple tests, and this increases the risk of identifying differences between groups by chance. The researchers say that rather than make stringent adjustments for this, they chose to report their associations to allow further investigations.

The study excluded workmates of the cases from the control group. It is unclear how this would affect results, as it would exclude at least a subset of people in the same occupations as the cases.

As the authors note, only occupational exposure to pesticides was assessed, not other methods of exposure, such as gardening as a hobby, residential exposure, wearing pesticide treated clothes, or dietary intake. Therefore it is not possible to say whether exposure through these routes affects the risk of parkinsonism.

Study suggests that using pesticides at work increases risk of Parkinson's three-fold.